The present invention relates to a steam heating device, and in particular to a device of this type which utilized for heating foods, especially those which are precooked and only require heating or freshening before they are served.
Many foods, such as hamburger buns, dinner rolls and the like, lose their freshness quite rapidly after they are baked. Since many restaurants, particularly fast food establishments, do not bake their rolls and breads on the premises, but purchase them in large quantities from independent bakeries, such bread products must be stored for several days before they are served. The bread products, even though they may be stored in relatively airtight wrappers, lose some of their moisture thereby acquiring a dry texture and stale taste.
It has been found that by subjecting breadstuffs, such as rolls, buns and pastries, to a blast of superheated steam, the food product is rapidly heated and much of its lost moisture restored so that it has a fresher taste. Many foods other than bread products can also be heated by steam, including those foods which are precooked and then frozen.
Typically, prior art food steamers comprise a closed compartment within which the food is contained, and steam is injected into the compartment through a plurality of orifices located either above or below the food. The steam is usually generated by injecting water onto the surface of a heated plate, which quickly heats the water to 212.degree. F. thereby converting it to steam. The steam is then passed through passages into a second chamber which is formed by at least one plate in thermal contact with the first-mentioned plate and which superheats the steam thereby raising its temperature above 212.degree.. From the superheating chamber, the steam travels through passages to the food compartment.
Many of the prior art food steamers of this type are formed with the steam generating chamber in the lower part of the unit and one or more steam superheating chambers stacked above the steam generating chamber. This results in a unit which is relatively large and bulky thereby occupying space which may be at a premium, particularly in fast food restaurants which are likely to have small food preparation areas. A further disadvantage to the stacked plate arrangement is that thermal conduction from one plate to the other may be poor. In most units, the heating elements are provided in only one plate, either the steam generating or steam superheating plate, and the plate which does not contain the elements may not attain a sufficiently high temperature to function properly. For example, if the heating elements are disposed in the steam superheating plate, which is typically positioned above the steam generating chamber, there may not be sufficient thermal conduction between the superheating plate and the lower plate forming the steam generating chamber to rapidly convert the water sprayed thereon to steam. If this arrangement is reversed and the heating elements are located in the lower, steam generating plate, the steam superheating plate may not become hot enough to superheat the steam to the desired temperature.
A still further disadvantage to the stacked plate arrangement is that there is a relatively large thermal mass, which requires long warmup times and radiates large amounts of heat to the ambient.
Although steam heating devices not employing the stacked plate arrangement are known, such as that disclosed in U.S. Pat. No. 3,794,016, their construction is such that superheating of the steam is not accomplished satisfactorily. In most prior art units, the steam is not superheated to a sufficiently high temperature and the moisture content of the steam, therefore, is quite high. In the case of bread products, the high moisture content of the steam imparts a wet film to the product, which is very undesirable. A further advantage to the use of steam which is superheated to a high temperature is that it heats the food more quickly.